Marine Biology: May 24, 2006
Why are there so many different types of reef fish?
I.
II.
III.
Introduction
A.
Many of the “big questions” in ecology are related to diversity, distribution and
abundance of organisms.
1.
The diversity of organisms on coral reefs is among the highest in the world.
B.
One of the most perplexing aspects of coral reef ecology is how animals so
seemingly similar, such as the many species of parrotfish or damselfish, can coexist
within a relatively small space.
1.
Appears to defy the “competitive exclusion” principle, which states that
“complete competitors cannot coexist”.
Some major families of reef fish
A.
Diversity among families,
B.
Similarities among species within a family.
1.
Example: Damselfish
Why are there so many families and species of coral reef fish? Several hypotheses
address this question.
A.
Hypotheses related to larval ecology
1.
Temporal and spatial variability in larval recruitment of different
species to the reef keeps a few species from dominating the reef. Factors
contributing to this variability:
a)
Currents are not consistent. May change in strength and direction
b)
Periodic large storms (somewhat stochastic in nature)
c)
Fish breeding at different times of day or year will be affected
differently by changes in currents and storms
d)
Different larvae have different rates of transport, time as larvae.

In different years, different larvae may be favored.
e)
The organisms co-occurring in the planktonic stage, such as predators,
may have a large impact on who makes it to the reef

Again, in different years, different larvae may be favored.
2.
Larvae of a potentially dominant species do not settle in numbers high
enough to competitively exclude other species.
a)
Because the population doesn’t remain local, the numbers can’t be
built up over generations.
B.
Hypotheses related to post-settlement ecology
1.
Niche partitioning/high level of specialization prevents competitive
exclusion.
a)
This appears to work on the level of Families (i.e. explains why there
are so many families). For example, with respect to feeding:

Parrotfish uniquely adapted: teeth fused into a beak allows
them to exploit the algae within coral

Butterflyfish utilize long snouts to pick at small denizens
within reef framework.

Goatfish have specialized barbels to pick through the sand.
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
2.
3.
Several species of wrasses are adapted as cleaner fish to pick at
parasites

Scorpionfish are lie-in-weight bottom predators, wellcamouflaged to fit in with their surroundings.

Damselfish “farm” algal territories and feed on associated
organisms.

NOTE: It is thought that tropical regions promote this niche
diversification due to their relatively stable year-round
conditions... Similar argument made for tropical rain forests.
b)
Nocturnal vs. diurnal feeders allows for further niche diversification
c)
This appears less true on the species level

Many seemingly similar species within a family

Example: Parrotfish of similar size, appearance and
location.

Subtle differences documented in time of activity, depth,
precise food preferences lend some support to niche
diversification idea.

Three very similar angelfish species have varying
amounts of sponges in their diets.
Predation prevents competitive exclusion.
a)
For generalist predator, species with higher densities may be
preferentially eaten, allowing for existence of more rare individuals,
or of species that might otherwise be competitively superior.
Disturbances occur frequently enough that superior competitors are
never able to dominate (=non-equilibrium hypothesis).
a)
Shown that “moderate” levels of disturbance promote highest levels
of diversity.
b)
Extreme disturbance, such as hurricanes may decrease diversity.

Glover’s example: Regions most thrashed by hurricane had
higher proportion of herbivores, while other regions less
disturbed had fewer.

Mosaic similar to the mussel bed example.
Study questions
1. Be able to state and briefly explain several hypotheses that have been put forth to explain
high levels of fish diversity on coral reefs.
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